Steering is the term applied to the collection of components, linkages, etc. which will allow for a vessel (ship, boat) or vehicle (car) to follow the desired course. An exception is the case of rail transport by which rail tracks combined together with railroad switches provide the steering function.
The most conventional steering arrangement is to turn the front wheels using a hand-operated steering wheel which is positioned in front of the driver, via the steering column, which may contain universal joints to allow it to deviate somewhat from a straight line. Other arrangements are sometimes found on different types of vehicles, for example, a tiller or rear-wheel steering. Tracked vehicles such as tanks usually employ differential steering—that is, the tracks are made to move at different speeds or even in opposite directions to bring about a change of course.
Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a sort of linear gear which meshes with the pinion, from side to side. This motion applies steering torque to the kingpins of the steered wheels via tie rods and a short lever arm called the steering arm.
The rack and pinion design has the advantages of a large degree of feedback and direct steering “feel”; it also does not normally have any backlash, or slack. A disadvantage is that it is not variable, so that when it does wear and develop lash, the only cure is replacement.
Older designs often use a recirculating ball mechanism, which is still found on trucks and utility vehicles. This is a variation on an older worm and sector design; the steering column turns a large screw (the “worm gear”) which meshes with a sector of a gear, causing it to rotate about its axis as the worm gear is turned; an arm attached to the axis of the sector moves a pitman arm, which is connected to steering linkage and thus steers the wheels. The recirculating ball adaptation of this design reduces the considerable friction by placing large ball bearings between the teeth of the worm and those of the screw; at either end of the apparatus, the balls exit from between the two pieces into a channel internal to the box which connects them with the other end of the apparatus—thus they are “recirculated”.
The recirculating ball mechanism also has the benefit of a much greater mechanical advantage, so that it was found on larger, heavier vehicles while the rack and pinion mechanism was originally limited to smaller and lighter vehicles. Due to the almost universal adoption of power steering, however, this is no longer an important advantage, leading to the increasing use of rack and pinion mechanisms on newer cars. The recirculating ball design also has a perceptible lash, or “dead spot” on center, where a minute turn of the steering wheel in either direction does not move the steering apparatus; this is easily variable via a screw on the end of the steering box to account for wear, but it cannot be entirely eliminated or the mechanism begins to wear very rapidly. This design is still in use in trucks and other large vehicles, where rapidity of steering and direct feel are less important than robustness, maintainability, and mechanical advantage. The much smaller degree of feedback with this design can also sometimes be an advantage; drivers of vehicles with rack and pinion steering can have their thumbs broken when a front wheel hits a bump, causing the steering wheel to kick to one side suddenly (leading to driving instructors telling students to keep their thumbs on the front of the steering wheel, rather than wrapping around the inside of the rim). This effect is even stronger with a heavy vehicle like a truck. Recirculating ball steering prevents this degree of feedback, just as it prevents desirable feedback under normal circumstances.
As vehicles have become heavier and switched to front wheel drive, the effort to turn the steering wheel manually has increased—often to the point where major physical exertion is required. To alleviate this, auto makers have developed power steering systems. There are two general types of power steering systems, hydraulic and electronic, though hydraulic-electric hybrid systems are also possible.
Hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel. Electric power steering (EPS) is more efficient than the hydraulic power steering, since the electric power steering motor only needs to provide assistance when the steering wheel is turned, whereas the hydraulic pump must run constantly. In EPS, the assist level is easily tunable to the vehicle type, road speed, and even driver preference. An added benefit is the elimination of environmental hazard posed by leakage and disposal of hydraulic power steering fluid.
An outgrowth of power steering is speed variable steering, where the steering is heavily assisted at low speed and lightly assisted at high speed. The auto makers perceive that motorists might need to make large steering inputs while maneuvering for parking, but not while traveling at high speed. The first vehicle with this feature was the Citroën SM with its Diravi layout, although rather than altering the amount of assistance as in modern power steering systems, it altered the pressure on a centering cam which made the steering wheel try to “spring” back to the straight-ahead position. Modern speed-variable power steering systems reduce the pressure fed to the ram as the speed increases, giving a more direct feel. This feature is gradually becoming commonplace across all new vehicles.
Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii.
A simple PRIOR ART approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle (see FIGS. 1A and 1B). The steering pivot points of a vehicle 1 are joined by a rigid bar 4 called a tie rod which can also be part of the steering mechanism, in the form of a rack and pinion for instance. With perfect Ackermann, at any angle of steering (e.g., as in FIG. 1B), the centre point 5 of all of the circles traced by all wheels will lie at a common point (e.g., where lines 6, 7, 8, which are respectively perpendicular to individual tires, meet). The centre point 5 may move toward the rear wheels of the vehicle 1 as the steering angle increases, and away from the rear wheels as steering angle decreases. In a vehicle where the Ackermann geometry is not perfect, lines 6, 7, 8 do not intersect in a single point 5. Perfect Ackermann allows for the least amount of wear on tires 2, 3, but may be difficult to arrange in practice with simple linkages. Accordingly, designers are advised to draw or analyze their steering systems over the full range of steering angles.
Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low speed maneuvers. Some race cars use reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tires while cornering at high speed. The use of such geometry helps reduce tire temperatures during high-speed cornering but compromises performance in low speed maneuvers.
The Ackermann geometry is commonly measured in terms of percentage, where a perfect Ackermann is measured as “100% Ackermann”, a steering geometry where the wheels are parallel at any steering angle—also called “parallel steering geometry”—is measured as “0% Ackermann”, and an Ackermann angle smaller than 0% is called as “Inverse”, “Reverse”, “Inverted” or even “Anti-Ackermann”. Along the same lines, a “negative Ackermann” corresponds to a decreasing Ackermann percentage and a “positive Ackermann” to an increasing Ackermann percentage. There is no exact appellation for an Ackermann exceeding 100%, as it corresponds to a geometry for which no use is usually seen, although it certainly brings further alterations to the dynamic motion of the vehicle.
For purposes of illustration, FIGS. 2A through 2G show a PRIOR ART vehicle 700 configured to have different steering geometries. More particularly, FIG. 2A shows vehicle 700 with wheels 701, 702 in a straight position and parallel with each other, as shown by equivalent distances A between parallel planes 703 and 704 that are in line with the wheels 701, 702.
FIG. 2B shows vehicle 700 with the steering wheel in a straight position and wheels 701 and 702 in a toe-in configuration, which is a setting where the front wheels of a vehicle converge when looking at the vehicle from the front.
FIG. 2C shows vehicle 700 with the steering wheel in a straight position and wheels 701 and 702 in a toe-out configuration, which is a setting where the front wheels of a vehicle diverge when looking at the vehicle from the front.
FIG. 2D shows vehicle 700 in a parallel Ackermann geometry configuration, where wheels 701 and 702 remain parallel when turning to the right, as shown by equivalent distances A between parallel planes 703B and 704B that are in line with the wheels 701, 702.
FIG. 2E shows vehicle 700 in a parallel Ackermann geometry configuration, where wheels 701 and 702 remain parallel when turning to the left, as shown by equivalent distances A between parallel planes 703C and 704C that are in line with wheels 701 and 702.
FIG. 2F shows vehicle 700 in a positive Ackermann configuration, which is a setting where the wheel on the inner side of the corner (which in this case is wheel 702), turns at a higher angle than the outside wheel (here wheel 701). The front wheels 701, 702 diverge when viewed from the front of the vehicle 700, notwithstanding the fact that the wheels 701, 702 may be parallel when the steering wheel is in the straight position.
FIG. 2G shows vehicle 700 in a reverse Ackermann configuration, which is a setting where the wheel on the inner side of the corner (which in this case is wheel 702), turns at a lower angle than the outside wheel (here wheel 701). The front wheels 701, 702 converge when viewed from the front of the vehicle 700, notwithstanding the fact that the wheels 701, 702 may be parallel when the steering wheel is in the straight position.
Some modern steering systems—like the ones used in Formula One racing for example—employ what is called a “variable ratio rack,” which is a rack having geometry designed to change the steering ratio in relation to the steering angle, meaning that to an increasing steering wheel angle corresponds a higher or lower—usually higher—angle of the wheels. These systems allow the driver to have less steering feedback while traveling on high speed turns, where the steering angle is small, and a higher feedback, where the wheels turn more in relation to the same angle variation, on slow corner where the steering angle is greater. Therefore, this system allows for a different ratio of steering between the steering angle of the steering wheel and the angle of the wheel on the ground, but that at the same time the system is fixed in nature, as the variable parameter is determined by the design of the rack and cannot be altered by the driver.
FIG. 3 shows a PRIOR ART open rack and pinion system 800 like the ones used in car racing. The system 800 includes pinion 801, rack 802, power steering system 803 being fed hydraulics through pipes 804, steering arm 805, and steering column connection hub 806.
FIG. 4 shows a PRIOR ART steering box 810 of the 2009 Formula 1 world champion contender Brawn GP race car. The steering column and pinion (collectively marked as 813) describes the view from the exterior of steering box 810 of the pinion location and steering column end which lay behind the outside cover mould. FIG. 4 also shows steering arms 811A, 811B and hydraulic pipes 812.